The flow of a fluid through a pipe or conduit is often measured in order to control the flow and/or monitor the amount of fluid passing through the conduit. Various methods exist for measuring flow of fluid through a conduit. These methods include a measurement of differential pressure across a fluid flow obstruction, utilization of magnetic flow meter (magmeter), and utilization of a vortex flow meter. These various devices and technologies generally employ different techniques in order to sense aspects of the fluid flow. In some cases, fluid velocity is measured and sometimes mass flow is calculated as well.
Measuring flow of a fluid using differential pressure measurement techniques generally includes utilization an obstruction device, such as an orifice plate, to partially obstruct the fluid flow. This partial obstruction generates a differential pressure between the upstream flow and the downstream flow. Measuring the differential pressure between locations upstream and downstream of the obstruction can provide an indication of flow. Generally, additional information is required in order for the differential pressure measurement to provide mass flow information. Specifically, information about the composition, pressure and temperature of the fluid must be known or measured. This is due, at least in part, to the fact that the differential pressure is not only based on the flow, but also is based on the fluid density, which itself may be a function of pressure and temperature. Further, the nature of the flow, laminar, transitional, or turbulent, may affect the differential pressure reading.
Vortex flow meters employ an operating principle based on the phenomenon of vortex shedding known as the von Karman effect. As fluid passes a bluff body, it separates and generates small eddies or vortices that are shed alternately along and behind each side of the bluff body. These vortices cause areas of fluctuating pressure that are detected by a sensor. The frequency of vortex generation is essentially proportional to fluid velocity.
Magnetic flow meters (magmeters) include a segment of pipe or conduit which measures the velocity of fluid passing through it. The flow meter creates a magnetic field in the fluid which induces an electromotive force (emf) or voltage in the fluid. The magnitude of the emf induced in the fluid is proportional to the velocity of the fluid flowing through the pipe. By measuring the induced emf, the magnetic flow meter measures the velocity of the fluid flow through the pipe. The flow meter creates the magnetic field by passing a current through coils encircling a pipe through which conductive fluid flows. The magnitude of the field is given by Ampere's law and is perpendicular to the flow of fluid through the pipe. Two electrodes, generally flush mounted on opposite sides of the flow pipe, measure the electric potential in the fluid.
In many process installations, a process fluid flows through a conduit, such as process piping. The process fluid may be a liquid, a gas, or a combination of both. In applications where the process fluid is a single homogenous phase (liquid, vapor, or gas) whose composition does not change with time, the calculation of flow parameters, such as mass flow, is relatively straightforward. However, in applications where the process fluid is not homogenous (such as with immiscible liquids), or whose composition changes with time, the calculation of flow parameters is more difficult. Examples of process fluids that generally have a liquid intermingled with a gas (e.g., multiphasic) include wet steam; mixtures of oil and natural gas; and mixtures of water, oil and natural gas.